Z slide cutting errors

[silverdog]

Hi,
My MechMate #70 have been heavily modified, transforming it from welded frame, to a bolted one.
I am happy with the results but a new problem occurred:
each sequential pass on thick board came "a little bigger" (a slight offset).
I cutted a 19mm MDF board in three 6.5mm pass, each pass is slightly bigger.
All of the sides have the same problem.
I suppose there is a little play on the v-bearings ( a thought I had writing this post :-) ) and the force on the mill push the cut on the outside ...
Any other ideas ?
Sergio

[bradm]

6mm tool? Have you tried cutting in the opposite direction? Or another finishing pass at full depth?

Tool flex, bearing play, mount flex are the ideas that come to mind.

[silverdog]

Thanks Brad ...
I checked the z axis and there was a little play due to the two upper v-bearing not properly tighted :-)

[silverdog]

.... but it was not the problem ...
My next check is to change the 6mm mill with a 8mm and see, then che software problems .....

[silverdog]

The 8mm mill cut perfectly ! (like never before) you can't see any sign of the multiple pass
So the problem was with the 6mm mill, I think. It wasn't mounted correctly on the chuck.

[bradm]

I've seen lots of flex with deep passes in 6mm and smaller tools. It always surprises me that they don't break.

[smreish]

ditto on what Brad said. I have a good deal of flex on the smaller dia bits when cutting. 5/16 (8mm) or larger seem to always yield better results for me.

[silverdog]

6mm one flute is my most used bit, I have used many times on much harder material than MDF and never had such an evident problems in 6 years.
The problem I had was that the mill was not properly mounted on the ER20 chuck, the same mill gaved no problem once properly mounted. It was my fault

[IamDave]

I cut mainly Extira and I use the 2 flute 1/4 and 5/16. The 5/16 really pulls it out and makes a clean cut. Extira I a heavy dust.